It is known that in the textile industry the problem of sensing fabrics discontinuity due, for example, to a seam or superimposition of fabrics of different characteristics, or even to variation of consistency of a same fabric, is of primary concern.
In particular, the problem is mostly present in the production of pantyhose and finishing of stockings. It is known, in fact, that a particularly important step of the process for making a pantyhose article is the one concerning the automatic positioning of the stockings before cutting them lengthwise. The positioning is made possible by detecting the garter line of each of the two stockings. The garter line being generated by the variation of fabric consistency in correspondence of the line of junction between the bodice and the stocking leg.
The known methods and devices for the detection of the garter line make use of means for driving the stocking on corresponding supports in order to temporarily stretch the fabric, that is, make it wrinkle-free, and allow a suitable optical sensor to sense the line.
The drawbacks deriving from the use of these methods and devices lie essentially in that they imply the need of stretching the fabric on the support means and, as these have a length less than the distance between the garter line and the elastic hem of the stocking, of firstly carrying the garter line over the support means and, after the detection thereof, bringing it back to a predetermined position outside and beyond the support means. This driving operation is time consuming also because of the sensor sensitivity.